Characterizing output for the Varian enhanced dynamic wedge field.
The output factor for an enhanced dynamic wedge (EDW) field, like that for a dynamic wedge field, is a complex function of the field dimension in the wedge direction. The large change in output for different field sizes (varying more than 40% for a 60 degrees wedge angle) is due to rescaling of the golden segmented treatment table (GSTT), which specifies the cumulative monitor unit weighting as a function of moving jaw position. The rescaling of the GSTT results in increased output on the central axis with a decrease in the value of the final moving jaw position in the wedge plane. The output factor (in air or in water) on the central axis of an EDW field can be predicted to within 1% by multiplying the output factor (in air or in water) of an open field of the same size with the ratio of the normalized GSTT (NGSTT) value at 4.5 cm, which corresponds to a 10 cm x 10 cm square field, to the NGSTT value at the final moving jaw position for the EDW field. Once the NGSTT factor is separated from the output for EDW fields, the field-size-dependent wedge factor varies less than 1%. This approach allows a simple and accurate determination of the output factor for rectangular and asymmetric EDW fields. The equivalent square method for determining output for rectangular fields applies to EDW fields with the same precision as it does to open fields. The output for EDW fields strongly depends on the final moving jaw position. Every 5-mm change in the final moving jaw position causes 3.5-5.4% error in monitor unit calculation.
Liu
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Department of Radiation Oncology, University of Florida College of Medicine, Gainesville 32610-0385, USA. liucr@bolus.health.ufl.edu
Li
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Palta
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D002138:Calibration; D008962:Models, Theoretical; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D013997:Time Factors